The present invention relates to catalysts for hydroformylation of olefins and to cobalt carbonyl complexes which possess a character capable of forming aldehydes selectively at low temperature and pressure and are easily separated and recovered after the reaction and recycled for repeated uses.
Chiefly used as catalysts known to date for use in hydroformylation (oxo reaction) of olefins are cobalt carbonyl, rhodium carbonyl and derivatives thereof. As hydroformylation is a reaction which is carried out in a liquid phase having the catalyst uniformly dispersed therein, separation of the catalyst from the reaction product is necessary after the reaction. In particular, when the hydroformylation is carried out by using a catalyst of cobalt carbonyl series, the separation of cobalt carbonyl from the product after the reaction has been a very important problem from the industrial point of view. In connection with the above, therefore, a number of patents such, for example, as U.S. Pat. Nos. 3,636,159 and 3,652,676 have been issued to date. However, the majority of the prior art methods for separating cobalt carbonyl is carried out by heating the product whereby cobalt carbonyl is thermally decomposed to precipitate metallic cobalt or by treating the product with an organic or inorganic acid to convert cobalt carbonyl into an oil-soluble, water-soluble or water-insoluble cobalt salt and subsequently separating the cobalt salt in an appropriate manner depending on the behavior of the cobalt salt. The metallic cobalt or cobalt salt separated by these methods is converted again to cobalt carbonyl in a separated step of the process and repeatedly used as catalyst for the oxo reaction. Otherwise, the metalic cobalt or cobalt salt may be recycled as such to the reaction system. In this case, a relatively severe condition (for example, 150.degree.-200.degree. C., 200-300 atm) is required since the oxo reaction would be ceased unless a condition which permits regeneration of cobalt carbonyl from the metallic cobalt or cobalt salt be selected in the oxo reaction vessel. Another method for the recovery of cobalt carbonyl comprises converting cobalt carbonyl into water-soluble NaCo(CO).sub.4 by using Na.sub.2 CO.sub.3, separating the conversion product in the form of an aqueous solution and having the separated aqueous solution reacted upon by sulfuric acid to generate HCo(CO).sub.4 which is then recycled as catalyst for the oxo reaction. In recent years, a method for separating cobalt carbonyl wherein the product is treated with a high molecular coordinating agent to convert the cobalt carbonyl into an insoluble solid which is then separated from the system has been taken up as a subject of study. However, all of the prior art methods described above inevitably require special steps for the recovery of the catalyst.
It is also known that a complex of cobalt carbonyl comprising a substituted pyridine with an electronegative non-hydrocarbon is a good catalyst for the oxo reaction (see U.S. Pat. No. 3,231,621). However, removal of the catalyst in this case requires a special measure.
The present inventor already proposed a complex of cobalt carbonyl and a pyridine compound as a catalyst for the oxo reaction (U.S. Pat. No. 3,996,164). However, this prior art required such a condition that the complex is previously prepared and then added to the reaction system. As a result of extensive researches made to overcome the drawbacks of these prior art catalysts and to improve separation characteristics and selectivity to aldehydes at low temperature and pressure, the present invention has been accomplished.